Rubber tires are often prepared in a manufacturing process by first building a tire carcass and then building a tire tread over the carcass. The tire tread is conventionally applied to the tire carcass as a usually contoured, uncured rubber strip which is wound around the carcass with the ends of the uncured rubber strip meeting to form a splice. The ends of the uncured rubber tread strip for a new tire are usually skived, or cut, at an angle to permit the ends of the uncured rubber strip to overlay (overlap) each other to some degree, instead of a straight cut in a form of a butt splice, to form a splice comprised of joined ends of the uncured rubber tread strip. Such tire tread fabrication utilizing a splice to join the ends of an uncured tread rubber strip is well known to those having skill in such art.
For such practice, the ends of the uncured rubber tread strip are desired to have a degree of tackiness, sometimes referred to as building tack, so that the tread splice holds together after its construction and is suitable for a subsequent tire building and cure. However, for such splice, the ends of the uncured rubber tread strip often do not have sufficient desirable natural building tack for such purpose so that a rubber cement is applied to at least one end of the rubber strip to aid in promoting building tack. For example, see U.S. Pat. No. 8,143,338.
Providing suitable building tack to an end of an uncured tread rubber strip is considered to be significantly more challenging where the rubber composition contains a high content of precipitated silica, namely a hydrophobated precipitated silica, where the content of such silica in the rubber composition significantly approaches or particularly exceeds the content of elastomer itself. Such high content of hydrophobated silica significantly is considered to reduce the building tack of the uncured rubber composition.
In practice, precipitated silica (an amorphous synthetic silica) is hydrophilic in nature and thereby challenging to efficiently blend and disperse within diene-based elastomers unless it is made more hydrophobic in nature as is well known to those having skill in such art. Such hydrophilic precipitated silica may, for example, be hydrophobated in situ within the rubber composition or may be pre-hydrophobated prior to its addition to a rubber composition. For example, see U.S. Pat. Nos. 4,474,908, 5,780,538, 6,127,468 and 6,573,324.
The precipitated silica may be hydrophobated for example, with one or more hydrophobation promoting compounds which are reactive with hydroxyl groups (e.g. silanol groups) contained on the precipitated silica which may be comprised of, for example, one or more of alkoxysilanes, alkylsilanes, halogenated alkylsilanes, and silica coupling agents such as, for example, bis(3-alkoxysilylalkyl)polysulfides containing an average of from about 2 to about 4 connecting sulfur atoms in their polysulfidic bridge ((e.g. comprised of bis(3-ethoxysilylpropyl)polysulfide), or an organoalkoxymercaptosilane.
It is understood that such hydrophobation promoting compounds rely, at least in part, upon reaction of its silane or siloxane moiety with hydroxyl groups (e.g. silanol groups) on the precipitated silica. For such reaction, it is understood that not all of the hydroxyl groups of the precipitated silica become chemically reacted with such compounds and that therefore a portion of the hydroxyl groups on the precipitated silica remain as available hydroxyl groups for further chemical reaction(s).
Historically, as previously indicated, an adhesive coating, sometimes referred to as a cement, is often applied to at least one face, or end surface, of the opposing ends of an uncured tire tread rubber strip to promote building tack and the ends joined to form a splice with the cement coating promoting suitable building tack between the ends of the uncured rubber tread strip to hold the tread splice together during the tire building process. For such procedure, a coating of a solvent based adhesive rubber composition, sometimes referred to as cement, is often applied to one, and sometimes both, faces of the opposing uncured tire tread ends. Alternatively, water based adhesives (cements) are sometimes used, but they require longer periods of time for drying than desired during the tire building process, to allow for water removal prior to the curing of the tire in a tire mold.
For this invention, it is desired to undertake providing building tack to the ends of such uncured rubber tread strip which contains a high content of particulate hydrophobated precipitated silica, namely an uncured tread rubber strip comprised of a rubber composition having a hydrophobated precipitated silica content at least about 70 weight percent, alternately at least about 105 weight percent and alternately at least about 175 weight percent based on its rubber content.
On such basis, then, it is desired for the building tack promoting cement to interact with the hydrophobated silica as well as the elastomer of the uncured rubber strip, particularly with the ends of the uncured rubber strip joined together to form a splice.
Historically, examples of various cements for such purpose and tread splices created by joining a ends of rubber composition, in general, may be referred to in, for example, and not intended to be limiting, one or more of U.S. Pat. Nos. 3,335,041; 3,421,565; 3,342,238; 3,514,423; 4,463,120, 4,539,365 and 8,143,338. It is readily observed that such exemplary cements are typically based on, for example, solvent solutions of compositions comprised of, for example, a base rubber, hydrocarbon oil, carbon black, tackifier resin and curative. However, no consideration is seen to have been given to adapting a rubber cement to promote building tack for ends of a tread rubber strip containing high content of hydrophobated precipitated silica which can be greater than the elastomer(s) contained in the rubber composition.
In the description of this invention, the terms “rubber” and “elastomer” may be used interchangeably, unless otherwise prescribed. The terms “rubber composition,” “compounded rubber” and “rubber compound” are used interchangeably to refer to rubber which has been blended or mixed with various ingredients and materials and such terms are well known to those having skill in the rubber mixing or rubber compounding art.